1. Field
This invention relates to simulated moving bed chromatographic separators. It is particularly directed to a process control procedure which improves the separation characteristics of a simulated moving bed.
2. State of the Art
Batch, continuous or simulated moving bed systems designed for the chromatographic separation of mixture components often consist of one or more beds of solid separator medium (sorbent). Sorbent bed systems representative of the types contemplated by this invention are disclosed in U.S. Pat. Nos. 2,985,589; 3,831,755; 4,400,278; 4,404,037; 4,011,113; 4,182,633; 4,247,636; 4,412,866; 4,501,814; and 4,511,476, the disclosures of which are incorporated by reference for their teachings concerning sorbent beds generally and the use of such beds in industrial scale operations.
Simulated moving bed (SMB) technology is well developed for applications involving separating the components of a fluid Typical applications of simulated moving bed chromatography include the separation of fructose from fructose-glucose solutions and the separation of sucrose from sugar beet or sugar cane syrups. Ion exchange resins are typically employed as sorbents for these applications. Solution components are differentially absorbed by the ion exchange resin so that a separation waveform develops within the simulated moving bed.
A typical simulated moving bed apparatus consists of several compartments (or individual columns) filled with solid sorbent. A fluid conduit interconnects the upstream and downstream ends of the system to form a loop through which fluid is continuously recirculated. The constant flow of fluid through the loop is called "internal recirculation flow." A manifold system of pipes and valves is provided selectively to position an inlet for feed material, an inlet for desorbent, an outlet for a sorbed component and an outlet for a nonsorbed (or less sorbed) component. Each inlet and outlet communicates with a separate bed compartment. Feed material enters the system at a designated compartment and is moved through the sorbent by the continuous internal recirculation flow. This moving contact results in a chromatographic separation of components. Sorbed component(s) which flow(s) at a relatively slow rate is removed from the sorbed component outlet. Nonsorbed component(s) which flow(s) at a relatively fast rate is removed from the nonsorbed component outlet. Desorbent is added at its inlet valve between the respective outlet valve positions of the sorbed and nonsorbed components.
At predetermined time intervals (step time) the designated inlet and outlet valve positions are displaced downstream one position on the manifold to the next sorbent bed compartment, which may be a discrete section of a vessel, (such as a column), or an individual such vessel, e.g., column. The step time is chosen such that the designation of valves is properly synchronized with the internal recirculation flow. Under these conditions the system eventually reaches a steady state with specific product characteristics appearing at predetermined intervals in sequence at each valve position. This type of system simulates valves held in a single position while the solid sorbent moves at a constant and continuous rate around the recirculation loop producing constant quality product at each valve.
The simulated version more closely approaches the character of an actual moving bed system as the number of compartments and valve positions increase. An important distinction between batch and simulated moving bed systems is that the internal recirculation flow is continuous in the simulated moving bed process. Except for very small adjustments to control internal pressure, the entering and exiting flow rates are continuous and constant, thereby approximating an actual moving bed system as closely as possible.
An equilibrated SMB system of the type disclosed by the aforementioned parent applications exhibits a steady state component separation waveform along the path of the recirculation loop. This waveform moves along the path of the recirculation loop with valve switching synchronized to maintain the desired steady state.